1. Field of the Invention
This invention relates to a location information service for a cellular telecommunications network, which enables individual mobile stations to receive information about their location within the network.
2. Description of the Prior Art
A number of proposals have been made to locate the position of mobile stations within a cellular telecommunications network. Each cell of the network corresponds to a particular geographical area and the cells are provided with individual identification codes. Thus, when a mobile station communicates through a particular cell, a rough estimate of its geographical location can be obtained using the cell identity. In urban situations, the cells may have an approximate diameter of the order of 200 meters.
A number of methods have been proposed to improve the locational accuracy. For example, the timing advance used in communications between the mobile station and the transmitter for the cell, can be monitored to determine whether the mobile station is relatively near to or far away from the transmitter. Also, schemes have been proposed to introduce an angular resolution around the transmitter. Other schemes involve a triangulation method using transmissions received from adjacent cells.
A location finding service is being offered commercially by Cell Point Systems AB of Kista, Sweden and reference is directed to WO 01/31966. The system is marketed under the name Finder™. Groups of subscribers can find out the location of members of the group from a mobile telephone. The user starts by defining a list of friends that they want to be able to locate, be seen by or communicate with. This involves a pre-registration procedure with the vendor of the Finder service. When a user wants to know the location of a friend, colleague or family member, the user initiates a request through their mobile telephone or computer and selects a name or names. The Finder system then locates both the user and the friend and calculates the distance and direction to the friend. The result is communicated back to the user as a text message.
A disadvantage of this system is that the user needs to pre-register with the system and only has access to the location of members of the relevant group.